Andrew M Brooks

Jan B Engelmann

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Journal Club
Editor’s Note: These short, critical reviews of recent papers in the Journal, written exclusively by graduate students or postdoctoral
fellows, are intended to summarize the important findings of the paper and provide additional insight and commentary. For more
information on the format and purpose of the Journal Club, please see http://www.jneurosci.org/misc/ifa_features.shtml.
Behavioral and Neural Effects of Delays during
Intertemporal Choice Are Independent of Probability
Jan B. Engelmann1 and Andrew M. Brooks2
1Department of Psychiatry and Behavioral Sciences, Emory University School of Medicine, and 2Graduate Program in Neuroscience, Emory University,
Atlanta, Georgia 30322
Review of Luhmann et al. (http://www.jneurosci.org/cgi/content/full/28/53/14459)
Decisions about consequences that are
delayed in time are referred to as inter-
temporal choice. The importance of such
decisions can easily be illustrated by their
pervasiveness throughout our lives: deci-
sions about education and career, pur-
chasing, saving and investing money, as
well as food intake, reproduction and re-
lationships involve tradeoffs between im-
mediate “consumption” and planning for
a better future. It is a well established find-
ing in economics and psychology that
people tend to prefer immediate over de-
layed rewards, even if larger sums are of-
fered after a waiting period. For instance,
most people would prefer to be paid $90
immediately, rather than $100 in 2 weeks,
a phenomenon referred to as temporal
discounting. In an attempt to explain such
subjective devaluation of future events,
economists and psychologists have mod-
eled discounting behaviors using discount
functions such as those illustrated in Fig-
ure 1. Although recent studies in neuro-
economics have begun to shed light on the
neurobiological basis of subjective valua-
tions during intertemporal choice (Mc-
Clure et al., 2004; Berns et al., 2006; Kable
and Glimcher, 2007), many questions about
the neural mechanisms of intertemporal
preferences still remain unanswered.
Intertemporal choice behavior can be
influenced by a number of factors, such as
the expected value (the product of proba-
bility and reward magnitude) of potential
outcomes of a choice and, importantly,
the length of the waiting period until an
outcome is realized. A number of re-
searchers have suggested that delays exert
their effect on behavior via enhancing in-
dividuals’ perceived risk of obtaining a re-
ward (Prelec and Loewenstein, 1991).
This explanation relies on the intuition
that a large number of unpredictable
events may occur between now and the
delayed receipt of reward, such as eco-
nomic downturns that may devalue
money, and even death. The question thus
remains whether the subjective effects of
delays are caused primarily by reducing
the perceived probability of an outcome,
or whether there are unique aspects inher-
ent to delays that influence intertemporal
choice.
A recent study in the Journal of Neuro-
science (Luhmann et al., 2008) sought to
answer this important question about in-
tertemporal choice by investigating par-
ticipants’ preferences for identical proba-
bilistic monetary rewards that were
presented either with or without temporal
delays until uncertainty resolution. To do
this, the authors developed a novel inter-
temporal choice task that dissociated de-
lay and probability. Participants, while
undergoing functional magnetic reso-
nance imaging (fMRI), made dichoto-
mous decisions about financial prospects
that varied in amount and probability.
Specifically, one option offered the op-
portunity to win a small cash reward at
varying levels of probability, whereas the
other option offered twice the amount,
but at relatively lower likelihoods. Impor-
tantly, decisions were made in the context
of two conditions, (1) an immediate and
(2) a delay condition. The immediate and
delay conditions were similar in that the
likelihood of obtaining financial reward
was represented visually by the number of
rectangles displayed below each option,
with an increasing rectangle number re-
flecting a decrease in likelihood. Impor-
tantly, the delay condition differed in that
rectangle number also determined the
length of the delay until the outcome was
revealed, while this occurred immediately
after response in the immediate condi-
tion. Trial durations were kept constant
across the two conditions, so that partici-
pants could not simply speed up the ex-
periment by choosing the smaller reward.
An analysis of the behavioral effects of
probability and delay on participants’
choice proportions revealed two interest-
ing results: (1) choices were influenced by
probability, such that participants chose
the smaller reward more often as proba-
bility of the larger reward decreased rela-
tive to that of the smaller reward, and, im-
portantly, (2) choices were influenced by
the delay, such that participants chose the
larger, less probable reward less frequently
Received March 2, 2009; revised March 30, 2009; accepted March 31, 2009.
This work was supported by a grant from the National Institute on Drug
Abuse to J.B.E. (T32 DA15040).
Correspondence should be addressed to Jan B. Engelmann, 101 Woo-
druff Circle, Suite 4304, Emory University School of Medicine, Atlanta, GA
30322. E-mail: jan.engelmann@emory.edu.
DOI:10.1523/JNEUROSCI.1037-09.2009
Copyright©2009SocietyforNeuroscience 0270-6474/09/296055-03$15.00/0
The Journal of Neuroscience, May 13, 2009 • 29(19):6055– 6057 • 6055
in the delay condition than in the imme-
diate condition. Of note, the latter effect
was observed despite identical probability
distributions and reward magnitudes be-
ing offered in immediate and delay condi-
tions, indicating that this effect is caused
by the delay alone. This finding is impor-
tant, because it suggests that simply delay-
ing the resolution of uncertainty about
consequences of decisions is perceived as
aversive by most participants, potentially
by increasing anxiety during the delay pe-
riod as suggested previously (Wu, 1999).
These behavioral results were paral-
leled by the neuroimaging results, which
revealed differential activation patterns
during the deliberation phase when par-
ticipants made choices in the delay com-
pared with the immediate condition. The
authors used parametric analyses to probe
for brain regions whose activity shows
correlations with (1) expected value and
(2) rectangle number of the chosen option
on each trial. Parametric analysis differs
from standard fMRI analysis in that it
identifies voxels whose activity shows a
linear relationship with a continuous vari-
able of interest, e.g., rectangle number or
expected value, by predicting the magni-
tude of hemodynamic responses based on
changes in the size of the underlying vari-
able on a trial-by-trial basis. This type of
analysis extracts the strength of the rela-
tionship between such a continuous vari-
able of interest and activity in a given
brain region. Of note are the authors’
findings related to rectangle number asso-
ciated with participants’ choices on each
trial, which probe for brain regions whose
activity shows significant correlations
with (1) probability alone in the immedi-
ate condition, and (2) probability plus de-
lay in the delay condition. Whereas the
authors report no significant correlations
with rectangle number in the immediate
condition, they demonstrated positive
correlations between rectangle number
and neural activity in the delay condition
in bilateral parahippocampal gyri, ante-
rior cingulate cortex, right superior pari-
etal lobe [Luhmann et al. (2008), their Fig.
4, top], and right intraparietal sulcus. To
further investigate the nature of these dif-
ferential activation patterns as a function
of delay, the authors conducted region of
interest (ROI) analyses. These revealed
greater correlations in the delay compared
with the immediate condition, as reflected
by greater regression coefficients, which
in the context of parametric modulation
represent the slope of the correlation be-
tween hemodynamic responses and num-
ber of rectangles [Luhmann et al. (2008),
their Fig. 4, bottom]. The fact that activity
in these regions showed significant para-
metric modulation by rectangle number
only in the delay condition underlines
their involvement in the purely temporal
aspects of intertemporal choice.
The authors furthermore observed an
intriguing heterogeneity in participants’
sensitivity to the delay. Previous research
in neuroeconomics has demonstrated
that individual differences at the behav-
ioral level can have powerful modulatory
effects on brain activations (Berns et al.,
2006). Participants in Luhmann et al.
(2008) exhibited varying attitudes to the
delay, ranging along a behavioral contin-
uum demonstrating aversion to the delay
(larger reward chosen less frequently in
delay compared with immediate condi-
tion) to delay indifference (larger reward
chosen with equal frequency in delay con-
dition). To capture these individual dif-
ferences in delay sensitivity, the authors
computed difference scores between the
probability of choosing the larger reward
in delay and immediate conditions for
each participant. The authors then corre-
lated behavioral delay sensitivity with its
neural equivalent, the difference between
activation during the delay and immedi-
ate conditions. Such investigations of in-
dividual differences are important for a
number of reasons: (1) past research in
the field of economics has largely ignored
these, (2) they can reveal important pat-
terns in the data that may be washed out
by standard fMRI analyses, and (3) they
directly link behavior to brain activations.
Significant correlations between behav-
ioral and neural sensitivity to the delay
were obtained in frontopolar cortex [Lu-
hmann et al. (2008), their Fig. 5]. In par-
ticular, the authors’ findings indicate a
negative relationship between neural ac-
tivity in frontopolar cortex and behavioral
delay sensitivity. This suggests that partic-
ipants whose choices were significantly
influenced by the delay (greater delay sen-
sitivity) recruited the frontopolar cortex
more actively. Together with previous
findings implicating the frontopolar cor-
tex in representing future events (Buckner
and Carroll, 2007), results from Luhmann
et al. are consistent with the notion that de-
lays exert their effect on intertemporal
choice via people’s ability to successfully
construct representations of future events, a
process referred to as prospection.
In summary, the study by Luhmann et
al. (2008) advances research on intertem-
poral choice by providing evidence for the
notion that unique aspects inherent to
temporal delays can significantly affect
behavior, and, in parallel manner, activa-
tion in a network of structures associated
with self-referential states. Importantly,
these effects of the delay were indepen-
dent of probability in the context of the
task used by Luhmann et al. (2008). The
authors interpret their results, showing
modulation of activity attributable to
temporal aspects of the task in regions
previously associated with self-projection
and prospection, as suggesting that one
important aspect of intertemporal
decision-making involves deliberations
Figure 1. Illustration of discounting functions commonly used to model intertemporal choice behavior (modified from Berns et
al., 2007). Discounted utility, the standard model of intertemporal choice in economics, assumes that people discount future gains
and losses in an exponential manner (solid line, functional form: t, where t represents time and  is the discount rate, here  
0.94). Hyperbolic discounting (dotted line) has been demonstrated to provide a better fit to most empirical data for animals and
humans [functional form: 1/(k * t1), here k0.15]. Both models demonstrate that the value of monetary incentives decreases
with increasing delay until receipt of reward. For illustration purposes, discount rates are exaggerated. Typical discount rates
observed in behavioral economics range between 10 and 25% over a 12 month period but can be larger depending on context.
6056 • J. Neurosci., May 13, 2009 • 29(19):6055– 6057 Engelmann and Brooks • Journal Club
about one’s future self. This notion is both
interesting and plausible, but the extent
and nature of the relationship between
prospection and temporal discounting,
which was not directly investigated by
Luhmann et al. (2008), needs to be ad-
dressed by future research. Finally, it will
be interesting to see whether results from
Luhmann et al. (2008) can be replicated
by studies investigating decisions involv-
ing extended time intervals (1 month)
(Fig. 1). The task of prospection becomes
more complex in such scenarios, and peo-
ple might rely on choice heuristics or re-
cruit affective processes to guide their de-
cisions when facing such increased
complexity. It is therefore likely that inter-
temporal decision-making about conse-
quences that are significantly delayed in
time will lead to greater recruitment of af-
fective centers, such as the anterior insula
and amygdala, which, somewhat surpris-
ingly, were not modulated by temporal as-
pects in the study by Luhmann et al.
(2008). This conjecture, however, re-
mains to be tested by future research.
References
Berns GS, Chappelow J, Cekic M, Zink CF, Pag-
noni G, Martin-Skurski ME (2006) Neuro-
biological substrates of dread. Science
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Berns GS, Laibson D, Loewenstein G (2007) In-
tertemporal choice–toward an integrative
framework. Trends Cogn Sci 11:482– 488.
Buckner RL, Carroll DC (2007) Self-projection
and the brain. Trends Cogn Sci 11:49 –57.
Kable JW, Glimcher PW (2007) The neural cor-
relates of subjective value during intertempo-
ral choice. Nat Neurosci 10:1625–1633.
Luhmann CC, Chun MM, Yi DJ, Lee D, Wang XJ
(2008) Neural dissociation of delay and un-
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Engelmann and Brooks • Journal Club J. Neurosci., May 13, 2009 • 29(19):6055– 6057 • 6057


Behavioral and Neural Effects of Delays during Intertemporal Choice Are Independent of Probability

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Behavioral and Neural Effects of Delays during Intertemporal Choice Are Independent of Probability

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